Past JS interop

This page addresses previous iterations of JS interop for Dart that are considered legacy. They are not deprecated yet, but will likely be in the future. Therefore, prefer using dart:js_interop going forwards and migrate usages of old interop libraries when possible. While dart:html and other web libraries are closely related, they're covered in the package:web page.



dart:js exposed a concrete object wrapper to interop with JS objects. This wrapper contained String-based methods to dynamically get, set, and call properties on the wrapped JS object. It was less performant due to wrapping costs and ergonomically more difficult to use. For example, you did not get code-completion as you couldn't declare interop members and instead relied on Strings. Many of the functionalities exposed in dart:js like allowInterop were later re-exposed through other interop libraries.

This library has been legacy ever since package:js and dart:js_util were released. It will likely be the first to be deprecated.



package:js introduced functionality to declare interop types and members. It allowed users to write interop classes instead of interop extension types. At runtime, these classes were erased to a type that is similar to dart:js_interop's JSObject.

class JSType {}

Users of package:js will find the syntax and semantics of dart:js_interop familiar. You may be able to migrate to dart:js_interop by replacing the class definition with an extension type and have it work in many cases.

There are significant differences, however:

  • package:js types could not be used to interop with browser APIs. dart:js_interop types can.
  • package:js allowed dynamic dispatch. This meant that if you casted the package:js type to dynamic and called an interop member on it, it would forward to the right member. This is no longer possible with dart:js_interop.
  • package:js' @JS has no soundness guarantees as return types of external members were not checked. dart:js_interop is sound.
  • package:js types could not rename instance members or have non-external members.
  • package:js types could subtype and be a supertype of non-interop classes. This was often used for mocks. With dart:js_interop, mocking is done by substituting the JS object instead. See the tutorial on mocking.
  • @anonymous types were a way to declare an interop type with an object literal constructor. dart:js_interop doesn't distinguish types this way and any external named-argument constructor is an object literal constructor.



Along with @JS and @anonymous, package:js later exposed @staticInterop, which was a prototype of interop extension types. It is as expressive and restrictive as dart:js_interop and was meant to be a transitory syntax until extension types were available.

@staticInterop types were implicitly erased to JSObject. It required users to declare all instance members in extensions so that only static semantics could be used, and had stronger soundness guarantees. Users could use it to interact with browser APIs, and it also allowed things like renaming and non-external members. Like interop extension types, it didn't have support for dynamic dispatch.

@staticInterop classes can almost always be migrated to an interop extension type by just changing the class to an extension type and removing the annotations.

dart:js_interop exposed @staticInterop (and @anonymous, but only if @staticInterop is also used) to support static interop semantics until extension types were added to the language. All such types should now be migrated to extension types.



dart:js_util supplied a number of utility functions that could not be declared in an package:js type or were necessary for values to be passed back and forth. This included members like:

  • allowInterop (which is now Function.toJS)
  • getProperty/setProperty/callMethod/callConstructor (which are now in dart:js_interop_unsafe)
  • Various JS operators
  • Type-checking helpers
  • Mocking support
  • And more.

dart:js_interop and dart:js_interop_unsafe contain these helpers now with possibly alternate syntax.